Over the past decade, there have been great developments in the field of image sensors, the commercialization of these products have led to increasing amount of consumer and industrial products that have revolutionized the areas of inventory, security, and photography in general. However, there are still problems with the quality of image sensors, and it is the development of solutions for these problems that will allow it to continue to rise in use and application.
One problem that has plagued image sensors is a concept known as “banding”, in particular one type of banding is known as the “first frame effect”. “Banding” occurs during an image capture, and results in a line or “band” of darker pixels appearing in the captured image. This problem can largely be attributed to the parasitic capacitance between the imaging pixel cell and the column signal line, and noise in the power source for the reset voltage, these types of noise are commonly referred to as “reset noise”.
The issue of banding has been dealt with before with some success through the use of a variation on double sampling, in combination with a memory device.
During a normal double sample, which is designed to reduce fixed pattern noise, the pixel is reset, allowed to integrate and then sampled at a sample time. Shortly thereafter, the pixel is reset again and sampled once again so that a second reset voltage for that pixel can be captured. The processing circuitry then compares the captured second reset voltage and the captured sample voltage, in order to determine an actual sample voltage free of the fixed pixel noise.
There is a variation on this technique in the prior art, which allows the imager to reduce “reset noise” as well as the fixed pixel noise. The double sample is performed in the normal manner, however there is a memory device attached to the row, which captures the first reset voltage. This first reset voltage can then be used to determine the noise between the first and second reset voltage, and that information can be used by the imaging process circuitry in order to remove that noise from the actual sample voltage.
In order to accommodate this method of reducing the “reset noise” a memory device must be added to imager. This type of solution focuses on the issue of noise in the voltage supply, but does not address in particular the issue of noise due to parasitic capacitance between the column signal line and the imaging cell.
The “banding” that results from parasitic capacitance, is typically referred to as the “first frame effect”. This type of banding creates a dark line of pixels from the first row that has been reset after the first sample signal has been placed on the column line. This distortion of pixel intensity makes the first frame of the imager unserviceable. In prior art systems this frame has simply been discarded with only the subsequent images being used for the purpose of imaging.
It may be considered that this is an inappropriate solution, as this adds time to the image capture cycle for the imager. Additionally it may be considered that this will lead to problems in some image capture systems, whereby a sequence of frames is being captured, and the first frame is needed in order to accomplish the task for the imager. For example, in a package transportation belt with a stationary mounted imager for decoding bar-codes, the belt speed may prevent a package from being properly scanned. Other examples would be apparent to one skilled in the art that the first frame should not be discarded.
It is to be noted that although a specific imaging architecture has been discussed to illustrate the deficiencies in the prior art, other imaging architectures could contain the same deficiencies. Thus, the problem discussed could occur in other circuits that use a similar technique for pixel readout.
Therefore, there is a need for a method and apparatus for improving image quality from an electronic image by reducing banding resulting from parasitic capacitance between the imaging pixel and the column signal line.